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The Bad Astronomer writes "A star in a galaxy 2.7 billion light years away wandered too close to a supermassive black hole and suffered the ultimate fate: it was literally torn apart by the black hole's gravity. The event was seen as a flash of ultraviolet light flaring 350 times brighter than the galaxy itself, slowly fading over time. Astronomers were able to determine that some of the star's material was eaten by the black hole, and some flung off into space. Although rare, this is the second time such a thing has been seen; the other was just last year."

At 3.5 billion miles the black hole is able to out-gravity a star of its own hydrogen atmosphere. Am I reading that right?

Yes, that's right. The way it happens is this: the star is in orbit around the black hole. The edge of the star closest to the black hole is in one orbit, and the opposite edge of the star is in another orbit. So they'd drift apart, if the star's gravity weren't holding them together. If this effect is large enough, then the star's gravity isn't enough to counteract it, and different parts of the star head off in their own separate orbits.

Your average stellar-mass black hole (the sort you get left over after some types of supernova) wouldn't be able to do this at 3.5 billion miles. But the black hole in this story is one of the supermassive ones you get at the centres of galaxies, with a mass 3,000,000x that of the sun. Also, the star in question is a red giant, which has a huge, puffy atmosphere (something like 0.2 billion miles across), which makes it easier to strip off: the opposite edges of it are in *very* different orbits around the black hole, so they pull apart more easily.

Say that the black hole has mass M, and the star has mass m. The radius of the star is r, and the distance between the black hole and the centre of the star is R. So the edge of the star closest to the black hole is at a distance of (R-r), and experiences a gravitational field from the black hole of:

G M / (R-r)^2

where G is the gravitational constant. (Don't worry - this will drop out eventually.) The edge of the star farthest from the black hole is at a distance of

Good maths and all but there's one thing you need to consider- If you're in stable orbit you don't actually fall inwards.

The sun for example has twice the pull on the moon as the earth (do the maths and see for yourself). It doesn't fall into the sun because it's in a stable orbit.

Likewise in this example. It's not a case of the black hole pulling more than the sun at a given distance. It can, but it's not all that relevant, plenty of orbiting bodies have more gravity pull from a nearby larger mass than they exert themselves but that's not what determines whether or not something gets pulled into the larger body.

What does determine whether or not something gets pulled into the larger body is if something disrupts the orbit. In this case the most likely culprit is charged particles from the event horizon stripping the sun of its outer layers.

Good maths and all but there's one thing you need to consider- If you're in stable orbit you don't actually fall inwards.

I think you've misunderstood my posts. I agree that a pointlike object in a circular orbit will remain in a circular orbit, absent any external factors. However, a non-pointlike object is actually in a range of orbits - different orbits for different parts of the object - and will drift apart unless held together. (Note that the separate parts, after this, will still be in orbit.) This is the same effect [wikipedia.org] that produces the tides on Earth; when an orbiting body is close enough to be torn apart by this effect, it's the Roche limit [wikipedia.org]. (You'll see a derivation equivalent to mine in both of those articles.)

Another way of thinking of it: as you say, it's not a case of the black hole pulling more than the star at a given distance. Instead, it's a matter of the difference between the pull of the black hole on different parts of the star being greater than the gravity of the star holding itself together.

I'm not sure where you got the idea that I was talking about the star being pulled into the black hole. That happens, certainly, but through a range of other effects: primarily, I would guess, through friction between the star and other material in orbit around the black hole. (You ascribed it to charged particles from closer to the event horizon, but these are emitted in jets perpendicular to the accretion disk, rather than omnidirectionally.)

Finally, I apologise for making an argument from authority, but I am an astronomer, though this isn't my exact field of research. I don't expect you to take my word for it, but I hope this will persuade you to read my posts in enough detail to understand the point you've missed.

Um, no orbits are caused by falling. The fact that the moon is in orbit around the earth doesn't mean that it is not also falling around the sun. Just because the earth is orbiting around the sun, doesn't mean it's not also in orbit around the galaxy.

Hmmm, also terminology you are using seems a bit wrong. The fact is that you aren't falling towards an object if you are in orbit because the horizontal velocity is such that even though you are being drawn together your horizontal position is keeping you from ever hitting. Again, you are always falling towards the object, stable orbit just means your horizontal velocity causes you to miss all the time.

If Sagitarius A* is a supermassive black hole, with a mass of, say, 4.1 million solar masses, its radius is probably no more than 6.25 light hours-- 45 AU. This star drifted within 5.2 light hours-- 37 AU.

"light hours" have the advantage of conveying just how large a light year is--63240 AU, or (if you must), 5.87 trillion miles.

Or, to put another way.

Space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.--Douglas Adams

At 3.5E billion miles, the gravitational acceleration from the black hole, which is about 3 million times the size of the sun is 12 meter per second squared, roughly the same as the Earth's gravity.

The star was described as being a red giant, using generous assumptions here favoring the star, the mass of the star being 10 times our own size and it's radius being 50 million miles, the surface gravity is about 0.2 meters per second squared, which is lower than Pluto's.

The star's orbital momentum helps it here since the acceleration due to this would roughly cancel out all of the gravitational acceleration, however this is at the center of mass, 50 million miles away from the surface. At this distance, the core of the star is only experiencing 97% of the gravity as the outside. That difference of 3% amounts to 0.36 meters / second squared of acceleration that was not canceled out at the surface.

Gravity alone doesn't selectively pick out hydrogen and leave helium behind. I'm guessing that's more the usual atmospheric escape when an object gets too close to a mass of charged, high velocity particles. Earth can't hold onto it's hydrogen against the suns solar wind. In this case a sun can't hold out against a black holes radiation.

Giant stars like this are layered, with the heaviest elements that are undergoing fusion in the center and lighter ones as you go outward. So the black hole ripped away the hydrogen because that was what was the farthest out and thus bound the weakest to the star.

It's not a matter of experiencing more gravity though. The sun has more pull on the moon than the earth. That doesn't mean the moon gets pulled into the sun. The hydrogen on the outer layers of this sun would still have more or less the same orbital velocity as the rest of the sun. There has to be something disrupting the orbit.

The hydrogen on the outer layers of this sun would still have more or less the same orbital velocity as the rest of the sun.

No it wouldn't, because the gravity gradient is very steep, and the star large. So the closer part of the star would like to have a substantially higher orbital velocity than the center of mass of the star, and would tend to want to be stripped off. A more massive (but equal size) star would hold onto its outer layers for longer. But once it comes too close, then the black hole's gravity (gradient) is simply too great and the star is ripped apart. Nothing needs to disturb it except said gravity.

The hydrogen on the outer layers of this sun would still have more or less the same orbital velocity as the rest of the sun. There has to be something disrupting the orbit.

First, tidal forces [wikipedia.org] will tear the star apart, then the resulting ring of gas will heat up due to friction (since layers closes to the hole will move faster), which causes the gas to radiate away its potential energy and spiral into the hole.

Gravity does pick out some elements more than others. The Boltzmann distribution of helium atoms tails out at a much lower velocity than the distribution of hydrogen atoms. Near the top of the atmosphere, you will see many more hydrogen atoms than helium atoms shooting outward at escape velocity. This is why on Earth, we only have a tiny bit of helium to fill our balloons that came from radioactive decay of heavy elements, and most of it will someday make it into space.

If you want to learn more about the phenomena this [wikipedia.org] might be a good place to start. That's the distance at which a satellite will be torn apart into a ring by the gravitational shear of its primary. First gasses, then liquids (both fluids, but liquid's higher density and stronger inter-molecular attraction would let it get closer), and finally, even rigid bodies will get torn apart. I would guess a star could be roughly modeled as a liquid body with a gasseous atmosphere. The formulas don't really accoun

You have to remember that the black hole has a mass millions of times that of the star. A pebble next to a mountain. And it didn't "out-gravity" it, gravity doesn't work that way. Your mass pulls the earth with the same force that the earth pulls on you. The star tugs at the black hole with the same force that the black hole tugs on the star, but the black hole is so big it no more notices the star than the earth notices an ant. There's already a comment explaining how the difference in gravity from one sid

I would think that the SC radius would indicate that relativistic gravity [for want of a better term] does not behave quite the same as classical gravity. Therefore, there will be some different gravitational effects.

Also, there's plenty that's lighter than hydrogen.

There's hydrogen nuclei, for example.There's electrons.There's quarks in that state of being ripped apart from each other, just as a new quark / antiquark pair is forming... which in normal physics would cancel out the old quark, leaving the n

I don't get it. You ask the question, and post a link to the answer? A quasar is a supermassive black hole at the center of a galaxy. This (what the Slashdot post refers to) is a more normal black hole that happens to be next to a star, and is going 'NOM NOM NOM' on it.

Events like this are rare; they probably only happen every 100,000 years or so per galaxy. So the astronomers looked at 100,000 galaxies, giving them good odds they’d see something like this once per year. Their gamble paid off.

Considering that it's an event that's estimated to occur once every hundred thousand years per galaxy, I'd say it's rare. The fact that they watched over 100,000 galaxies, and got 1 per year as estimated does not diminish the sheer volume of galaxies watched nor decrease the rarity of the event.

My understanding of this event is that it required a black hole from the center of the galaxy, such as Sgr A* in the center of the milky way. My guess, is that the jets would be faced perpendicular to the galaxy, so we would see them from the side. It might be bright, but from so far away in our galaxy and not head on, I doubt it would have an effect on us. I am more concerned about Betelgeuse going supernova as it is rather close.

Reading the article I am struck by just how little fact and data this is based on. This is something that happened 2.7 billion light years away and this is one possible explanation for what happened. I have no idea of how likely it is to be the correct explanation, but I didn't read anything that told me that it was the only explanation.

With that in mind, I am happy to float the possibility that this flash in a very distant galaxy, very long ago, was actually a mega-strike in a intergalaxial war. I have abs

We'll never get to witness that, either Sol will become a red giant first, consuming anything that still lives on the earth, or, the gravity of the black hole with eat the earth before Sol succumbs. Either way, we'll already be dead.

We'll never get to witness that, either Sol will become a red giant first, consuming anything that still lives on the earth, or, the gravity of the black hole with eat the earth before Sol succumbs. Either way, we'll already be dead.

Uh, antimatter is seen all the time. Heck, the "P" in "PET scan" stands for "positron", the electron's antiparticle. As for dark matter, it's "seen" in gravitational effects, which is admittedly indirect and somewhat inconclusive. Still, humans are rather biased. The matter you're made out of is mostly quarks and electrons. Quarks are affected by all four fundamental forces: (G)ravity, (E)lectromagnetism, (W)eak, and (S)trong. Electrons are only affected by GEW. Neutrinos have just GW and are therefore hard to detect. Maybe there's matter that's just affected by G; it would only show up on cosmological scales like dark matter seems to.

Who knows? Maybe there's a whole segment of matter humans are unfamiliar with which interacts very little with the matter we know about but interacts with itself in complicated ways. Maybe there are dark matter solar systems populated by dark matter people who are just as confused as we are about the weird gravitational anomalies caused by our otherwise invisible existence. Communicating through gravity would certainly be an interesting challenge! I don't really believe this, but my point is basically the same as Hamlet's: "There are more things in heaven and earth than are dreamt of in your philosophy"--that is, it's arrogant to expect humans to be in a position to observe all the parts of the universe. Perhaps some things are just hidden.

I'll grant you the antimatter issue, but I still like my tongue-in-cheek jab at the GGP for saying that dark matter wouldn't be "seen" after falling into a black hole. It is "dark" after all, meaning it cannot be seen in the human sense of the word, so the difference between it being in a black hole and not being in a black hole is visibly none.

Of course, the really interesting thing is that it's possible that the actual act of falling into a black hole is the only thing that would ever make dark matter visible. So it would never be seen before, or after, but possibly could be seen *during* its descent into the singularity.

It occurred to me because we know that normal matter emits gamma radiation as it falls into the black hole, but not knowing the mechanism that causes it means it might be done by all matter, including dark matter. There is probably evidence already disproving this idea, but I thought it was interesting nonetheless. It would have a certain poetic quality, though, if the only time dark matter were visible was during its disappearance.

So far, the only force that we know dark matter responds to is gravity. If it's only influenced by gravity, then it's hard to see how it would build up the nice, complex chemistry that the electromagnetic force produces, which allows our form of life to exist.

Of course, it's possible that there are other forces that affect dark matter (and don't affect regular matter). There are ways we could detect this: for example, if two clumps of dark matter hit each other, then they should pass right through each ot

Does anyone ever wonder if antimatter is our representation of what exists as matter on the other side of any given (or perhaps all) black hole(s) inside another dimension/universe/whatever you wanna call it? Universe pairs? Hawking theorized that black holes have white hole pairs - maybe his math just indicated that there is no Lord Nibbler poo at the completion of a black hole (or the start of our universe) but rather another instance of er...space
ie- how does a singularity occur w/ infinite mass (or

No, gravity *is* energy. We view it as potential energy, but all energy is carried by particles [up to gravitons, which we are still looking for, but have not found yet].

The energy of gravitational wells has the ability to warp space, as is found in special relativity, and confirmed by experiments.

Now, I am not a physicist either: I have a degree in engineering [and did take physics, and have a family of physicists]. However, I am going to propose that I've long thought that gravity is a by-product of sp

No. Black holes like the one being talked about do not loose much energy to gravitational waves. In order to dissipate energy via gravitational waves the mass must accelerate. So a pair of masses orbiting each other will shed gravitational energy, a galactic black hole sitting in the center of the galaxy does not move much and so does not emit gravity waves. Regarding Hawing radiation dissipation, the temperature of the Hawking radiation is greater as the mass of the BH is smaller. In order to loose net ma

Gravity isn't an energy dissipation at all. Other forms of radiation are from energy consumption (xrays, gamma radiation, even light) but gravity is closer to being a dynamic property of how mass interacts with space-time.

I'd be much more interested in knowing how the antimatter+matter collision energy gets held back within a black hole absorbing both.

Here are some relevant bits of physics you might not be aware of: * White holes are somewhat shaky. From their Wikipedia article,

However, this region does not exist for black holes that have formed through gravitational collapse, nor are there any known physical processes through which a white hole could be formed.

There are apparently solutions to the Einstein field equations giving black/white hole pairs, but black holes do not need white holes to exist.
* There is an interpretation of antimatter in quantum field theory as matter traveling backwards through time (that phrase is very imprecise unfortunately). More details here [stackexchange.com]; I'm not qualified to really discuss it as I'm jus

I did mean to put that I am a total um "not a physics guy" just the armchair piece-together-what-i-can-repeat dreamer type.

I know:). I don't think any physics people would use the phrase "Lord Nibbler poo at the completion of a black hole" or make the mistake of thinking a black hole has infinite mass and/or violates conservation of mass. But thank you for your honesty regardless.

Black holes don't have infinite mass. Just many stars' worth of mass, squeezed into a very tiny area. Relativity tells us that the *density* is infinite, but quantum mechanics shows that even the singularity must have some volume. The mass doesn't go anywhere - in fact, things that fall into the black hole never actually make it to the center due to time dilation effects.

And, antimatter is just like normal matter, it just carries an opposite charge because it's made from different versions of subatomic part

GP said "most powerful" which is not synonymous with strongest. For example, conspiracy theories aside, the US president is probably one of the single most powerful men on the planet, but it's a matter of force multiplication, in a test of strength I'd bet on most any bodybuilder that challenged him.

In the case of gravity it's more a matter of force division. The nuclear forces fall off very rapidly with distance, becoming effectively nonexistent at even molecular scales. Magnetism fairs better, but stil

The nuclear force is a bit weird as it's due to quantum-mechanical interactions between the quarks of the associated hadrons. I don't pretend to really understand it, but they have some lovely diagrams and animations on wikipedia [wikipedia.org] that will leave you convinced that either we're misunderstanding what's going on or God has a wacky sense of humor.

Short answer, the nuclear force is a very strong attractor at around one femtometer, becomes repulsive at distances less than 0.7 femtometers, and decreases to insign

It's actually a residual force that originates with the strong interaction between quarks, which is a force that reaches a strength of about 10,000N at a limiting distance of roughly the size of a hadron, and then remains constant regardless of how much farther apart the coupled quarks move

If I understood the portion of Feynman's QED lecture on more modern physics, this is because unlike the chargeless photon, the gluon actually has color charge and thus binds to the very force it is mediating, right?

Well, we've created antimatter in the lab and it seems to behave very much like normal matter, it just has the opposite charge (for protons/electrons) and Baryon number (a QM property). So I suspect it would behave very much like normal matter, in fact I doubt we can actually tell whether a celestial object/event involves matter or antimatter, though it seems fairly likely that all the "native" matter in a particular galaxy will be the same type, otherwise it would have mutually annihilated whenever a gas cloud of one kind interacted with it's opposite, though a matter galaxy could conceivably capture a rogue star from a passing antimatter galaxy - as long as the rogue star never exploded or hit something directly it would likely be indistinguishable except for a *very* faint and diffuse halo where its antimatter-based solar wind contacted and annihilated the interstellar medium.

Dark matter though... that's an interesting question. As far as we can tell it only interacts gravitationally so it will never glow or collide with anything, since both are EM interactions. The Bullet Cluster would seem to indicate that it even passes right through other dark matter. Which raises an interesting question, while it could presumably be sucked into a black hole's event horizon it might continue to behave just as bizarrely, possibly even being able to escape again somehow. We just have no idea what the stuff is, it's even possible that it's not matter at all, but rather a phenomena symptomatic of a fundamental misunderstanding of the nature of reality, much as black-box radiation in the 1800s led to the development of quantum mechanics and radically altered our understanding of the universe. It was widely believed at the time that we basically understood everything about physics, with just a few loose ends still to tie up (BB radiation, the cause of spectral lines, and a couple others). Instead those loose ends led to the unraveling of virtually everything we thought we knew and opened the door to something far stranger.

There's also the possibility that black holes don't exist at all and the question is nonsensical. We have evidence of ultra-massive non-luminous objects, but little if any for the existence of the defining characteristic of black holes, an event horizon. We assume they are black holes because our theories say that anything that massive would collapse into a singularity, but think about it - we're postulating that a body can become so dense that it creates a region of space where the laws of physics themselves to break down! There are several competing theories that make such a situation impossible, one that I like is based on the fact that Einstein treated gravity as a special case - all other energy fields generate a gravitational field based on their energy density. Einstein felt that it would be "double dipping" to have gravitational fields do so and discarded the idea. However, if we rework the equations assuming that they do in fact do so then we find that as the gravitational field strength becomes extreme the "secondary" gravity generated by the extreme energy density of the "primary" field pulls back against the primary source, causing the field strength to plateau at a level less than that required to create an event horizon, regardless of the density of the central object. If that, or some other mechanism, puts an upper limit on gravitational field strength it seems likely that the ultramassive objects are simply some sort of exotic quark-degenerate matter that happens to be non-luminous. As far as I can remember photons are radiated when (1) charges accelerate through space (as with radio transmissions), (2) electrons descend to a lower orital, and (3) nuclear processes result in lower binding energies. I don't know much QM, but it seems likely that (4) quark bindings and transmutaions that result in "left-over" energy would be a final source, and the only one that might apply to a neutron star, which are apparently directly observable (I couldn't find much in the way of de

IANAP, but I'd postulate that, assuming dark matter obeys the laws of time dilation, it would behave exatly as normal matter beyon the Schwartzschild radius of a black hole. Time would stop due to extreme time dilation and thus the particle would effectively stop at the Schwartzschild radius. Then the black hole would be a tiny bit more massive, the Schwartzschild radius would be a tiny bit bigger.

Dark matter, like any mass is affected by gravity, so there is likely great gobs of the stuff in orbit around galactic black holes. The issue with pulling in dark matter is the problem that dark matter does not interact strongly with anything, including itself. So there is no mechanism for the DM to loose angular momentum which would have to happen for it to fall in. So the only dark matter that gets eaten is the stuff that is on a trajectory that intersects the event horizon area of the BH. The reason that

I apologize if it's a dumb question, but isn't the whole point of a black hole that not even light escapes?

The gravity tore apart the star before it entered the black hole. Watching all the videos about black holes and space might lead one to think that orbits are easy to achieve, but after I ran some particle simulations [google.com] using simple Newtonian physics in my game engine, I noticed that most particles will slingshot around a source of heavier gravity when they approach, and be flung too far away for gravity to recapture it. In a stellar nursery this sling shot effect places a limit on the star's size, the other main contributing factor being initial density of the nebula. This is true for black holes as well as planets or asteroids approaching a star. So, although some of the star will fall into the black hole, a lot more of it gets flung away from the black hole -- It's a classic case of Conservation of angular momentum...

They're seeing what happens when something gets close to a black hole, not goes into it. You can see things "going into a black hole" before they've reached the event horizon. Also: In my sim, elliptical orbits that didn't result in the object being flung away became tighter and rounder orbits over time.

That schools don't have kids play with simple sims like these in class is Ridiculous! My high-school age little brother hasn't played a traditional game in three weeks. Since I gave him the gravity sim (particle engine stress test) to play with -- all he does is simulate solar systems and formation of stars, or big stars eating little stars, etc. It's the first time I've ever seen him interested in space beyond the Halo Universe! He asked me about Quantum Physics yesterday!